Method of material handling with automatic guided vehicles

ABSTRACT

A system and method of automatic guided vehicles that are capable of providing synchronized travel along a line or path such that regular manufacturing operations may be performed to material or workpieces on the vehicle without the need for a traditional conveyor systems.

TECHNICAL FIELD

The present invention is generally directed to material handlingvehicles and more particularly to a system and method of automaticguided vehicles that are capable of providing synchronized travel alonga line or path such that regular manufacturing operations may beperformed to material or workpieces on the vehicle without the need fora traditional conveyor systems.

BACKGROUND OF THE INVENTION

For over a century now, manufacturers have used assembly lines toprovide reliable and consistent work flow of workpieces and materialthrough various manufacturing operations to create an end product. Theseassembly lines widely vary depending on the desired end product as wellas the type of manufacturing process; however, almost all have somecommon features. Most assembly lines include a conveyor system such as achain conveyor system, power and free conveyor system or any other typeof material conveyor system that is designed and installed permanentlyinto the facility. To provide consistent work flow, most conveyorsystems are configured to couple to or support a workpiece at asubstantially uniform predetermined distance and configured to movealong a path at a set speed. Each conveyor system is configured to keepthe workpiece consistently spaced no matter the speed, acceleration,deceleration, stop, or start conditions. As all workpieces are securelycoupled in some form together, consistent travel of all workpiecesautomatically occurs. Most conveyor systems also require a chain, belt,or track that forms the path, couples all objects together and isinstalled permanently into the manufacturing facility. As such,traditional assembly lines and conveyor systems work extremely well atproviding consistent through put of work in manufacturing operations,especially where the timing of workpieces entering and exiting aparticular work station is important, but they have been generallyexpensive to initially install and also lack flexibility for easyreconfiguration. The presence of the conveyor equipment often preventsaccess to the part or workpiece from all sides and prevents workers fromeasily and safely crossing the conveyor path.

Automatic guided vehicles or AGVs are commonly used in many industriesto provide material handling and transport various loads without a humanoperator. The term “AGV” is commonly used to refer to robust vehicledesigns having any number of available automated guidance systems. Theterm “AGC” is also commonly used to refer to less robust vehicles suchas automatic guided carts which are similar in nature to AGVs, however,are typically designed to carry smaller loads. Throughout thisapplication, including the claims, the term “AGV” or automatic guidedvehicle shall mean and include both AGVs and AGCs as well as any othervehicle that is capable of being autonomously guided. Autonomousguidance and AGVs do not include vehicles being remotely controlled byhuman operators, but instead must be capable of following a path orroute without human intervention.

Current AGV designs generally include a frame with at least two wheels,one of which may be a drive wheel. The drive wheel provides motion tothe cart and may also be a steerable drive wheel but in some instances,the non-driven wheels may instead or in combination, act as thesteerable wheel. An AGV requires a guidance system to control itsmovement. A variety of guidance systems are available for use in AGVsincluding wire guidance, laser guidance, magnetic tape guidance,odometer guidance, inertial guidance, dead-reckoning, optical guidanceand a variety of other less used guidance systems. Each type of guidancesystem generally has associated positives and negatives. For example, aninertial guidance system may be susceptible to tracking errors where thetravel distance and direction measured by the AGV differs from theactual distance and direction of travel due to wheel slip on thesupporting surface. A variety of methods have been proposed to minimizesuch tracking errors but the tracking errors may compound over longtravel distances. As such, many AGVs include backup or secondaryguidance systems which may provide a position or status check, and assuch be used to correct for any errors. For example, way point referencemarkers may be added to the system such as magnetic paint, radiofrequency identifier tabs and optical tags to allow an AGV to update itsposition to a correct position and thereby minimize any guidance errors.Some AGV systems today that use sensors that detect existingenvironmental features and do not require the addition of referencemarkers.

Due to the variety of potential errors introduced by at least one of theguidance and drive systems, AGVs have primarily been used in facilitiesonly for the moving of materials such as delivery of raw materials to anassembly line, the removal of finished materials to storage, and fromstorage to distribution and shipping. In these instances, the AGV may beprogrammed with a specific path that an individual AGV travels along,but none of the issues associated with a conveyor system in amanufacturing operation are of concern. In addition, while AGVs may bepart of material handling system and work in cooperation with theoverall system, they do not individually coordinate movement in thefacilities other than avoiding potential collisions between AGVs. Assuch, AGVs have generally not worked in coordination but instead eachperform their own unique task and only coordinate to prevent collisions,or move material along desired paths such that parts A are coordinatedto arrive with a parts B at a particular work station.

Some manufacturers have tried to use automatic guided vehicles inmanufacturing operations or in various facilities as a replacement fortypical conveyor systems although until the present invention, nomanufacturer has successfully implemented such a system. Coordinatedmovement of AGVs in a cost-effective and reliable manner, similar toconveyor systems was not yet possible. For example, if an AGV system wasto coordinate all AGVs using a central broadcast time signal, the timesignal to each AGV has complications with respect to starting andstopping, which is frequently required in a manufacturing facility. Morespecifically, there are many timing problems associated with identifyingthe exact time a vehicle stops or starts due to inherent latencies incommunication systems. Without an exact time the vehicle stops orstarts, it is unknown where a particular vehicle is in relation to othervehicles and in relation to the external manufacturing operation. Assuch, problems may occur in restarting the system, such with spacingbetween the AGVs.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method of automaticguided vehicles that are capable of providing synchronized travel alonga line or path such that regular manufacturing operations may beperformed to material or workpieces on the vehicle without the need fora traditional conveyor systems.

The method of operating a conveyor system for material handling in afacility generally uses a plurality of AGVs, each having a controller.The controller includes a communication system and a guidance system.The AGVs may be in communication with a central controller, at least oneAGV, such as the lead AGV may be in communication with the centralcontroller, or the system may use a distributed control system withoutthe central controller, with the control residing collectively in theAGV controllers.

The method generally includes the steps of determining a travel pathhaving at least one line portion defined by an initial line and an endline; inputting a guided route for the AGV to follow, wherein the guidedroute includes instructions regarding the AGVs travel along the at leastone line portion; assigning a unique ID to each AGV; guiding theplurality of AGVs along the travel path; automatically determining alead AGV in the line portion; automatically determining a last AGV inthe line portion; automatically updating the last AGV in the lineportion each time an AGV from the plurality of AGVs crosses the initialline of the line portion; automatically updating the lead AGV to theimmediate trailing AGV in the line portion each time the lead AGV fromthe plurality of AGVs crosses the end line; and maintaining a designatedspacing between each AGV from the plurality of AGVs within the lineportion.

The method may further include a step of inputting the travel path,including the line portion, into each guidance system of the pluralityof AGVs. Of course, the input travel path may be only input in the leadAGV and then distributed as needed to the other AGVs. The method mayalso include inputting a desired line speed for the AGV to travel alongthe line portion. The method may also include a step of inputting adesired travel speed for the AGV when the AGV is not in the lineportion. The desired line speed and the desired travel speed may bedifferent, such that an AGV may progress through a certain line segmentat a reduced pace while manufacturing operations are being performed andthen travel between line segments, or back to the entrance of the sameline segment as quickly as possible to reduce the number of AGVs used ina particular facility or manufacturing operation.

The method may use a lead AGV and be configured such that each of thefollowing plurality of AGVs in the line portion match the lead AGVspeed. The method may automatically determine the lead AGV and assign alead token to the lead AGV. Of course, the method may automaticallyupdate the lead AGV by passing a lead token from the lead AGV as itcrosses the end line to the next trailing AGV. Similar to the lead tokendescribed above, the method may automatically determine a last AGV andassign an end token to the last AGV. Of course, the method mayautomatically update the last AGV in the line portion each time an AGVfrom the plurality of AGVs crosses the initial line of the line portionfurther includes the step of passing the end token from the precedinglast AGV to the updated last AGV. The method may include automaticallyupdating the last AGV in the line portion by providing the unique ID ofthe AGV updated as the new last AGV to at least one of another AGV and acentral controller.

The method step of guiding the plurality of AGVs along the continuoustravel path may include the step of guiding the AGVs through the lineportion, and further the step of guiding the plurality of AGVs withconsistent spacing and speed in the line portion. More specifically, thestep of guiding the AGVs through the line portion may include the stepof tracking the distance traveled by the AGV since crossing the initialline to enter the line portion. The AGV when it crosses the end line ofthe line portion may clear the distance traveled from memory.

The method maintains a designated spacing between each AGV and mayfurther include the step of determining the distance traveled from theinitial line on the line portion for each AGV. As such, the system mayensure that the distance between each AGV is determined by subtractingthe distance traveled of a trailing AGV from a prior AGV and comparingthe determined distance between matches the designated spacing. Morespecifically, the step of ensuring that the distance between matches thedesignated spacing may further include the step of adjusting the speedof the trailing AGV to approach the designated spacing and repeating thestep of subtracting the distance traveled of the trailing AGV on whichspeed was adjusted from the prior AGV and adjusting the speed, andrepeating the steps of subtracting and adjusting until the trailing AGVand prior AGV are spaced apart with the designated spacing. The step ofadjusting the speed may further include the step of limiting any speedadjustment within a specified range. As the step of adjusting the speedfor AGV is performed, each subsequent AGV may automatically, in responseto a speed adjustment by a prior AGV or any AGV in a line segmentperforms the steps of ensuring that the distance between each AGV is thedesignated spacing and adjusting speed as needed to maintain thedesignated spacing with the prior AGV.

The step of maintaining the designated spacing may further include thestep of continuously calculating the distance traveled by each AGV fromthe initial line along the line portion and communicating the distancetraveled to one of a prior and a trailing AGV. The step of maintainingmay further include the step of at least one of the prior and thetrailing AGV calculating the spacing between the two AGVs and whereinthe trailing AGV uses the calculated spacing to adjust the speed of thetrailing AGV to approach the desired designated spacing.

The method may include a step of stopping at least one AGV in the lineportion in response to a stop condition and wherein the stopped AGVcommunicates a stop status to each subsequent AGV within the lineportion and wherein each subsequent AGV stops upon receiving said stopstatus from any prior AGV. Upon stopping, each stopped AGV determinesthe distance to at least the immediate prior AGV. The leading AGV of thestopped AGV may automatically restart upon removal of the stop conditionand communicate a restart signal to other stopped AGVs, including atleast the trailing AGV upon starting. Upon receiving a restart signal,the trailing AGV would determine a start delay. The step of determininga start delay may include the step of determining a distance between theprior AGV and the subject AGV and if the distance is greater than thedesignated spacing a start delay of zero is determined and if thedetermined distance is less than the designated distance a start delayof sufficient time to ensure at least the designated spacing ismaintained to the prior AGV is determined. At least each subsequent AGVwould perform the steps of determining a start delay upon receiving arestart signal from the prior AGV.

In the method, the step of maintaining a designated distance may furtherinclude the step of automatically determining the length of at least oneof a carrier and a load on the AGV and adjusting the spacing between theAGVs to match the designated distance while accounting for variations inlength of the load and carrier.

The method may include the step of maintaining a communication linkbetween each prior and each trailing AGV and stopping a trailing AGVupon failure of the communication link.

The travel path may be a continuous loop and the method may include astep of directing the AGV along the travel path to the next initial lineafter the AGV crosses the end line of the current line portion.

Further scope and applicability of the present invention will becomeapparent from the following detailed description, claims, and drawings.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given here below, the appended claims, and theaccompanying drawings in which:

FIG. 1 is a schematic view of an exemplary conveyor system usingautomatic guided vehicles;

FIG. 2 is a side view of automatic guided vehicles within a portion ofan exemplary conveyor system; and

FIG. 3 is a schematic drawing of an exemplary AGV controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is generally directed to material handlingvehicles and more particularly to a system and method of automaticguided vehicles that are capable of providing synchronized travel alonga path such that regular manufacturing operations may be performed tomaterial or workpieces on such automatic guided vehicles without theneed for traditional conveyor systems.

The system 10, as illustrated in FIGS. 1 and 2, generally includes atravel path 20 having individual line segments or portions 22 bound byan initial line 30 and an end line 40. As an automatic guided vehicle(AGV) 50 travels along the path 20 within the line segment 22, each AGV50 maintains a separation distance 60 from adjacent AGVs. With any givenline segment 22 with at least two AGVs, there is a lead AGV 52, asubsequent AGV 54 and a last AGV 58. Of course, in some instances thesubsequent AGV 54 and last AGV 58 may be the same. The system 10 mayinclude a central controller 70 having a communication system 72communicating with each AGV 50. However, in some systems 10, no centralcontroller 70 or communication system 72 is used and instead each AGV 50may communicate with each other independently in a distributedcommunication system or in addition to the central controller 70. EachAGV 50 may include an AGV controller 90 generally having a controlmodule 74, a guidance module 76, and a sensor module 82 coupled to atleast one sensor 84. The guidance module 76 may include a traveldistance encoder module 80. The controller 90 may be in communicationwith an antennae 78 communicating with at least one of the centralcontrollers 70 and an adjacent AGV 50, or any other AGV 50 in the travelpath 20. It is expected that each AGV have wheels 56 of which one may bea drive wheel and one a steerable wheel.

The method generally includes providing a system 10 having AGVs 50configured to travel along a determined travel path 20. The system 10 isgenerally configured to work in and around the facility such as amanufacturing, distribution, or warehouse facility. It is particularlysuited to a manufacturing facility in that the system is configured toallow for consistent spacing and speed as it moves along the lineportion 22 or in some instances between individual work stations atwhich manufacturing operations may be performed.

The travel path 20 and included line segments 22, as well as the initialline 30 and end line 40 of such line segments 22, are provided and inputinto the AGV controller, specifically the guidance controller 76. Onceeach AGV 50 has learned the desired travel path 20 including linesegments 22, a unique ID is assigned. During operation, a centralcontroller or a distributed control between the AGVs 50 may guide theplurality of AGVs 50 along the travel path 20. The system 10automatically determines a lead AGV 52 in the line portion 22 as well asa last AGV 58 in the line portion 22. As each AGV 50 continues to movealong the line segment 22, at some time the lead AGV 52 will cross theend line 40 of the line segment 22 and the system 10 will automaticallyupdate which AGV 50 is the lead AGV 52 in the line segment 22. Inaddition, as AGVs continuously enter the line segment 22, the systemautomatically updates which is the last AGV 58 in a particular linesegment 22. As the AGVs 50 progress along the line segment 22, each AGV50 maintains a designated spacing between each AGV 50 forming theplurality of AGVs 50 in the line segment 22. Of course, the line segment22 may be broken into multiple sub-segments in 24 which the spacing mayvary between manufacturing operations. However, in each instance withina particular line segment 22 or sub-segment 24, which could beconsidered just smaller line segments 22, the spacing is maintained as adesignated spacing 60 between each AGV 50.

The step of determining a travel path 20 having at least one lineportion 22 defined by initial line and an end line is generallyperformed in the manufacturing facility, warehouse or distributioncenter. The terms manufacturing facility, warehouse, distribution centeror other building in which the AGV operates in the present inventionhereinafter shall be generally referred to as a facility. It is expectedthat an AGV will operate in and about a particular facility. As such,the operator of a system 10 will generally set up the facility similarto when a conveyor system is used between individual work stations.However, instead of installing a traditional conveyor system, theoperator of the facility determines a travel path 20 for AGVs 50 totravel between particular work stations or destinations. It is expectedthat the travel path 20 determined is a substantially continuous travelpath such that each AGV 50 after exiting a particular line portion 22within the travel path 20 eventually circles back to enter the initialline portion 30 within particular line segment 22. As the travel path 20is virtually determined in many instances, the only changes to afacility needed in moving a travel path may be as simple as providingnew initial or end lines 30, 40 by painting on the floor of thefacility. As such, the present invention provides all the positives of atraditional conveyor system, but is much more flexible and has very lowcost in making any changes in the travel paths 20.

Although not illustrated in the Figures, each travel path 20 may have avariety of different segments 22, 24 and be much more complex than thesimple loop route illustrated in FIG. 1. Once the travel path 20 isdetermined, each line portion 22 is defined by an initial line 30 and anend line 40. As illustrated in FIG. 1, the travel path 20 includesmultiple line portions 22 and in some instances, the initial line 30 andend line 40 are the same line such that an AGV 50 crossing the end line40 of one line segment 22 is also crossing the initial line 30 of asubsequent line segment 22. In such instances, the adjacent lineportions or segments 22 may be considered all one large line segment orportion 22 with sub-segments 24. It should be recognized that theillustrated travel path 20 and line segments 22 are only exemplary andsuch travel path and line segments 20, 22 will vary widely dependingupon the type of facility, layout of the facility, type of manufacturingoperations, travel paths selected to avoid people and any other desiredconsiderations. It should also be recognized that a particular facilitymay have multiple travel paths 20 such that a particular group of AGVsmay stay on a first travel path while a second group may stay on asecond travel path, although the system is so flexible such that AGVscould be easily added or taken away depending upon the need in any onegiven travel path.

Once the travel path 20 and line portions 22 are determined, they areinput into at least one of a central controller 70 or AGV controller 90.The input may be simply a program upload or in some instances may be alearning system wherein the AGV 50 is manually directed along the path20. While the initial line 30 and end line 40 forming a particular lineportion 22 may be completely virtual and programmed into an AGVcontroller 90, it is expected that in most instances such an initialline 30 and end line 40 will be physically delineated within thefacility. More specifically, it is expected that the sensor module 82and sensors 84 may determine and sense the initial line 30 and end line40. By providing actual, physical markers within the facility such asoptical markers or magnetic tape on the supporting surface, thepotential for errors is minimized and it ensures that each AGV 50 iscapable of tracking the distance or time since the AGV 50 crossed theinitial line 30 within a particular line segment 22 in a consistent andreliable manner. Any number of known techniques may be used to input theguided route for the AGV to follow into one of the controllers 70, 90.It should be recognized in some systems 10 the system 10 will notinclude a central controller 70 but may be distributed and controlledamongst the individual AGVs 50.

Each AGV 50 will be assigned a unique ID. The unique ID may behard-built into the AGV 50 by the manufacturer or may be virtuallyassigned when each AGV 50 is added to a particular travel path 20. Theunique ID allows each system 10 to easily determine which AGV 50 is in aparticular line segment 22 as well as communicate spacing and distancetraveled over an initial line segment 30. In addition, the unique ID maybe used in determining the lead AGV 52, subsequent AGVs 54 and last AGV58 in a particular line segment 22. The unique ID assigned to each AGV50 may also be used in associating a lead token with the lead AGV 52 aswell as a last token with the last AGV 58. These tokens then may beautomatically updated as described below as AGVs cross in and out of aparticular line segment 22.

During operation, the system 10 may guide the plurality of AGVs alongthe travel path and in and out of particular line segments 22. Theguiding of individual AGVs 50 along the travel path 20 may be done byany known method including methods such as inertial guidance,dead-reckoning, magnetic systems including magnetic tape, markers, paintor guide wires, optical guidance systems, or any other type of guidancesystem. It is expected that markers or other instructions needed for anyparticular guidance system will be added by the facility as needed forthat particular guidance system. The additional markers added aremarkers specifically delineating the start and end of line segmentsparticularly the initial line 30 and end line 40 of particular linesegments unless the particular guidance system supports virtual markers,however, a virtual initial line 30 and a virtual end line 40 must thenbe. As such, it is expected that if an AGV uses an inertial guidancewhich is free from external sensor inputs other than accelerometers andwheel encoders, an optical or magnetic sensor will form one of thesensors 84 and be used to determine the added initial and end lines 30,40.

When the system 10 is operational, the system 10 automaticallydetermines the lead AGV 52 in a particular line portion 22 as well asthe last AGV 58 in a line portion 22. With the lead AGV 52 and last AGV58 determined, the order of each subsequent AGV 54 following the leadAGV 52 is also known. The unique IDs assigned above allow the system 10,even if it is a distributed control system, to easily and readilydetermine the order of AGVs along a travel path 20 and in particular theorder of the AGVs within a particular line segment 22.

With the system 10 knowing the location of each AGV 50 within aparticular line segment 22, as the AGVs 50 progress along the travelpath 20 and a lead AGV 52 crosses the end line 40, the system 10 willautomatically update the lead AGV 52 to correspond to the immediatesubsequent or trailing AGV 54. As such, the immediate subsequent ortrailing AGV 54 becomes the lead AGV 52. Similarly, the system 10automatically updates the last AGV in the line portion 22 each time anAGV 50 from the plurality of AGVs 50 in a particular travel path 20crosses the initial line 30 of the line portion 22. The system 10 mayuse a lead token which is transferred from AGV 50 to AGV 50 such asbeing associated with each unique ID as well as a last token which isalso transferred between each AGV 50 as a new AGV 50 becomes the lastAGV 58. It is important to note that the system 10 automatically updatesthe last 58 and lead 52 AGVs which is always changing in a particularline segment 22.

To provide consistent through-put, the system 10 automatically maintainsa designated space 60 between each AGV 50 from the plurality of AGVs 50within the line portion 22. In addition, it is expected that the system10 will maintain a consistent speed between each of the AGVs 50 in aline portion 22. A central controller 70 may control the AGVs 50 tomaintain the designated spacing 60, however, each AGV 50 mayindividually control the spacing 60 between it and the prior AGV. Morespecifically, a prior AGV 50 may communicate with a subsequent AGV 54its distance traveled from the initial line 30, which the subsequent AGV54 compares at that time to its distance traveled from the initial line30 to determine the designated space 60 between the AGVs. If the spacingdoes not match the preset designated spacing, the AGV 50 may adjust itsspeed and continue the process of cycling communication, determination,and adjusting the speed until the designated spacing 60 is maintained.

It is expected that while inputting the travel path and line segmentsinto the guidance system of an AGV 50, the desired line speed for theAGV to travel along any particular line portion 22 is also input. Assuch, both the designated spacing and line speed may be predeterminedfor a particular line portion 22 such that the AGVs 50 act similar to atraditional conveyor system and are capable of replacing a traditionalconveyor system. In addition, a variety of travel speeds may exist suchas the AGV 50 in one line segment 22 traveling faster than the AGV 50 ina subsequent line segment 22. Furthermore, in areas of the travel path20 which are not line segments 22, it may be desirable to quickly movethe AGV from the end line 40 of one line segment 22 to the start orinitial line 30 of another line segment 22. By increasing the speed ofthe AGVs 50 in areas that do not form part of the line portion 22, thenumber of AGVs needed are minimized as an AGV may quickly exit aparticular line portion 22 and travel with increased speed travel to thestart of the next line portion 22. Of course, consistent speed may bekept through the whole travel path.

Even though the speed of a particular AGV in a line segment 22 istypically predetermined, or should match a set speed or speed range, thelead AGV 52 in any particular line segment 22 indirectly sets the actualspeed of subsequent AGVs 54. By the lead AGV 52 indirectly setting theactual speed of the subsequent AGVs 54, each AGV following matches thelead AGV's speed. More specifically, the AGVs maintain the desiredspacing 60 between each AGV 50 in a line portion 22 and as such, thespeed of the individual AGVs is substantially matched. Only the lead AGV52 attempts to follow the set or predetermined speed while allsubsequent AGVs 54 try to indirectly match the speed of the lead AGV 52by maintaining the designated spacing 60. More specifically, while thelead AGV 52 generally attempts to match its preprogrammed speed for itsparticular position along a travel path and particularly along a lineportion 22, some variations occur and to ensure designated spacing 60between subsequent AGVs 54, the subsequent AGVs only match the lead AGVsspeed by maintaining designated spacing 60. More specifically, thesubsequent AGVs 54 do not specifically match the lead AGVs' 52 speed andthe lead AGV does not communicate its speed to other AGVs, but bymaintaining the designated spacing 60, the speed of the lead AGV 52 isindirectly matched with minor variations. The designated spacing 60 iseasier to match than speed as an encoder on a wheel may easily track thedistance traveled from the point where the AGV entered the line segment22 for each AGV. As the initial line 30 is physically delineated, thelead AGV 52 and subsequent AGVs 54 use the distance traveled incalculating the spacing, which is more consistent than speed for anygiven AGV. Therefore, while the lead AGV 52 generally sets the speed forsubsequent AGVs, the system in reality is using the distance traveled asa comparison of spacing 60 and any adjustments of speed in subsequentAGVs 54 are to adjust the spacing 60 not generally to match the speed ofthe previous AGV. However, to maintain spacing, the subsequent AGVs 54will need to match the speed of the prior AGV as well as lead AGV 52 andwill do so indirectly through calculating the distance between the AGVs.As such, the system 10 uses very little computational power as it onlytracks the distance traveled by the AGVs since crossing the initial line30 to enter the line portion 22. Therefore, after an AGV 50 crosses theend line 40 of a particular line portion 22, the distance traveled iscleared from the memory of the AGV because it is either in a new linesegment 22 or it is in a travel path 20 between line segments orportions. Therefore, as each AGV 50 travels along a particular lineportion 22, an AGV determines its distance traveled and communicates itto at least the subsequent AGV 54. The subsequent AGV 54 then takes itsown distance traveled and compares it to the distance traveled to theprior AGV to determine the current distance between such AGVs and if thedetermined distance matches the designated spacing 60 desired at thatposition along the line portion 22. If the designated spacing 60 doesnot match the actual spacing, the subsequent AGV 54 will adjust itsspeed slightly to either increase or decrease the speed and as such,decrease or increase the distance between such AGVs. The system 10continuously cycles by communicating from one AGV 50 to at least thesubsequent AGV 54 the distance traveled, receiving a communication ofthe distance traveled by the prior AGV, determining its own distancetraveled, comparing it to the distance traveled of the prior AGV andadjusting speed as necessary. Adjustments in speed may be limited suchthat an AGV does not go faster or slower than a desired range within aparticular line portion 22. In addition, the adjustments in speed may belimited as the designated spacing is approached such that the AGV is notconstantly overcorrecting. Therefore, a range of error is allowed oneach side of the designated spacing. This range may be determined andvary widely between systems. In each event, the system, in particulareach AGV 50, attempts to match the designated spacing 60 by continuouslycalculating the designated spacing using distance traveled for eachadjacent AGV adjusting as needed and then performing the calculationsagain. The type of operations being performed, type of facility, andspacing 60 designated between each AGV 50 may all affect how often thesystem 10 needs to cycle its calculations.

It should be noted that as the AGV 50 uses distance traveled incalculating and maintaining the designated spacing 60, wheel slip mayoccur thereby providing false readings on the distance traveled. Inaddition, while the present invention refers to the distance traveledwith an encoder on the wheels, other systems may be used to calculatethe distance traveled, such as optical markers or any other knownsystem. As most AGVs include an obstacle avoidance system such asdetecting unexpected objects in the path, which many times are peoplecrossing the path of the AGV, such systems may also be used to ensurethat the AGVs do not collide in the event that any particular AGV in theline portion 22 has an incorrect distance traveled, which leads thesubsequent AGVs 54 to believe that the prior AGV 50 is located a greaterdistance away than its current actual distance. Therefore, the obstacleavoidance system may be used to ensure adjacent AGVs do not collide if amistake occurs in the travel distance, as the travel distance is used toadjust the designated spacing 60 between individual AGVs.

The use of a travel distance is particularly beneficial in maintainingdesignated spacing in the event of a stopped condition. A stoppedcondition may occur from a variety of events such as an error on amachine in the facility such that all of the AGVs subsequent to aparticular work station must stop, a breakdown of an AGV, or even aperson crossing the path of the AGV. As AGVs are already communicatingdistance traveled, a particular AGV may detect that the AGV ahead of itis stopped by the fact that the distance between it and the AGV ahead isshrinking. The trailing AGV will slow to maintain spacing. A minimumallowed spacing is set for the particular AGV/load/segment combination.When the distance between the AGV and the AGV ahead fall to or belowthat minimum distance the AGV stops. Each subsequent AGV in the segmentsimilarly slows and stops. In some systems, it is desirable that allAGVs in the segment remain within proper spacing or a very small rangein spacing. In some systems, it is desirable to stop the entire linewhen any AGV stops. As AGVs are already communicating distance traveled,a particular AGV may communicate to all AGVs or all subsequent AGVs inthe segment the stop status upon stopping in response to a stoppedcondition.

All AGVs 54 in the particular segment or all subsequent AGVs, as soon asa stopped status is received from any prior AGV, would stop until itreceives a restart signal from the originally stopped AGV. Upon theremoval of a stopped condition of the any stopped AGV particularly afirst stopped AGV, it would communicate a restart signal to all of theAGVs in the same segment. This restart signal may be communicated downthe complete line of AGVs in a particular line segment or communicatedfrom AGV to AGV such that a prior AGV only communicates with thesubsequent AGV 54 such that once the subsequent AGV restarts, it thensends a restart signal to the next subsequent AGV. In any stoppedcondition, it is likely that many of the AGVs need to stop quickly andas such, the designated spacing between particular AGVs may vary in astopped condition due to timing in receiving the stopped signal or evendifferences in the braking capabilities of AGVs, loads carried or otherconditions. Once each AGV is stopped or in response to a restart signal,the subsequent AGV calculates the designated spacing between adjacentAGVs and determines if any adjustment is needed during the starting orrestart procedure to maintain the designated spacing. For example, if aprior AGV entered a stopped condition due to a person stepping the pathof the AGV, it would stop quickly and at the same time send a stopsignal to the subsequent AGV. Upon receiving the stopped signal, thesubsequent AGV would stop, however, minor communication delays may causethe subsequent AGV to reduce the designated spacing to an amount lessthan desired. Therefore, when a restart signal is sent, the AGVreceiving the restart signal would calculate the distance to the priorAGV during the stopped condition and determine if a start delay isneeded.

If the determined distance between the adjacent AGVs is greater than orapproximately equal to the desired maintained spacing 60, the AGV wouldstart as soon as receiving restart signal as the start delay would bezero. However, if the spacing 60 is less than what is desired, the AGVwould calculate a start delay which upon starting would place it in thedesignated spacing 60 to minimize further adjustments needed. Therefore,upon exiting a stopped condition and receiving a restart signal, thestart delay is automatically calculated and adjusts so that upon restartthe AGV is immediately within the designated spacing. Therefore, astarting procedure for a plurality of AGVs has little effect on thesystem and only minor adjustments are needed once each AGV isoperational and traveling down the travel path 20. However, it isexpected that due to variances in AGVs and loads, some AGVs mayaccelerate faster than other AGVs, even if the acceleration is limitedin a restart condition. Therefore, the AGV, upon restart, would asdescribed above, determine the distance between the two adjacent AGVsand adjusting its speed as necessary to approach the desired spacing 60.It should also be noted that if an AGV 50 in the center of a lineportion experiences a stopped condition, the prior AGVs may continue onwithout stopping and as each AGV exits the line portion, such as passingthrough the end line 40, the lead token is passed from one AGV toanother and as such, the first stopped AGV may receive the lead token asthe last AGV exits from the moving group of AGVs. If an AGV experiencesa stopped condition, once the designated spacing grows beyond a certaindistance with a prior AGV, the stopped AGV, upon restarting may also beassigned a lead token such that two AGVs within the line portion areacting as lead AGVs so that the second lead AGV does not try to catch upto the last AGV in the group with the original lead AGV.

The system may also be configured to automatically receive and adjustspacing depending upon the particular loads carried by an AGV. In someinstances, a load carried by an AGV such as a tugger or forklift mayvary in length and such length may be provided to the AGV automaticallyand the AGV may adjust automatically the desired designated spacing 60such that it accounts for variations in the length of the load and thecarrier. Therefore, the AGVs may maintain the designated spacing betweenadjacent AGVs irrespective of the type of load or length of a particularload or even variations between the AGVs used on a particular linesegment.

As the AGVs communicate distance traveled and use such communicateddistance to determine and maintain the designated spacing 60 betweenparticular AGVs 50, upon a failure of communication by a particular AGV,a variety of steps may be taken. First, a particular AGV that issubsequent to the AGV with the communication failure may stop. Second,the AGV with the communication failure may stop as it is unable toreceive distance traveled from a prior AGV. Third, the AGV may useexternal markers to update the position. Fourth, the AGV with thecommunication failure may be configured to automatically leave the linesegment and travel path and travel to a repair area. Similarly, the AGVmay be manually removed from the segment. For example, the system mayautomatically determine that the AGV with the communication failure isno longer responsive and the unique ID is removed from the system and assuch, system controller, or the distributed controller determines thennew order for the AGVs in the segment. With the new order, each AGVperforms its function of lead AGV, subsequent AGV, and last AGV tomaintain proper spacing. In view of the above, the system 10 mayautomatically remove an AGV 50 and adjust the spacing 60 as neededbetween the remaining AGVs such that the operations being performed inany particular line portion 22 do not need to stop due to a simplecommunication failure. When an AGV breaks down, operators remove the AGVfrom the path. The system automatically performs the steps above tore-sequence the vehicles to allow a restart.

Therefore, during operation in the present invention, the first AGV 52which is the lead AGV generally acts as a pace car to the subsequentAGVs. The first AGV 52 travels through the line segment or portion 22 atthe desired speed and the second AGV in line or subsequent AGV 54follows the lead AGV 52 at the desired spacing distance. Therefore, thesecond or subsequent AGV 54 to the lead AGV 52 and each one thereafteris allowed to travel at the nominal speed but with enough speedmodulation and range in the allowed speed to allow it to track andmaintain a desired distance 60 behind the lead AGV 52 or any prior AGVs.

Therefore, the system 10 is aware of a particular AGV's 50 position onthe line segment 22, which is accomplished as described above using asensor that detects the initial line 30 and then detects the distancetraveled along the line segment 22 since crossing the initial line 30.The sensor 84 used to detect the initial line 30 may be any type sensor,such as proximity switch, limit switch, photo cell, magnetic sensor, barcode reader, RFID sensor, or any other sensor capable of detecting anappropriate target that senses or provides data that the AGV is crossingthe initial line 30. The sensors 84 may also be used to update theguidance system to targets along the line segment. However, a simple wayto determine distance from the line segment would be a second sensorsuch as an encoder attached to the wheel of the AGV, preferably atrailing wheel and not a drive wheel, such that the encoder provides thedistance traveled as the wheel turns.

As the system 10 needs to communicate distance traveled between each AGV50, the system generally 10 includes the communication system 72 asdescribed above. The communication may be centrally controlled such thateach AGV communicates only with a central controller 70 or may bedistributed such that each AGV communicates directly with other AGVs inthe system 10. However, the communication works, the AGVs communicatetheir position on the line segment 22 by communicating the distancetraveled from the initial line 30. By communicating the distancetraveled and any unique ID of a particular AGV, the relationship of eachAGV on a line segment 22 may be determined such that a centralcontroller may determine the order of the AGVs on the line segment andthe distance between each AGV or each AGV may determine suchinformation. If the communication is distributed, such as only at eachAGV level, each AGV may determine the position of the AGV ahead of it byscanning the distances reported by each of the other AGVs in thesegment. As described above, the unique vehicle ID or token may beassigned to each AGV allowing it to communicate its position on the lineand in particular, the tokens may be useful in determining the lead andlast AGVs 52, 58. Therefore, when a particular AGV enters a line segment22 by crossing an initial line 30, the communication system may querythe ID of the last vehicle to cross and provide it with a last vehicletoken while the former last AGV either clears or transfers such token.Similarly, the lead AGV token may be transferred between AGVs.

As each AGV broadcasts its position on the line segment, particularlyits distance traveled, each AGV may determine the spacing between it andthe prior AGV. The AGV controller 90 may use known control methods suchas PID to modulate its speed and maintain a constant distance betweenitself and the prior AGV. In the event a particular AGV stops due tofailure, operator intervention or other stopped conditions, thecontroller of the stopped AGV may send a signal to the other AGVcontrollers to stop and every AGV currently on the line segment 22 orjust subsequent AGVs may stop. When the AGV restarts, a similarcommunicated signal from the restarted vehicle to the other AGVsprovides a restart.

As described above when AGVs stop, communication delays, equipmentfailures, or just differences between particular AGVs may cause each AGVto lose its proper designated spacing 60. To control spacing and preventcollision, the system uses the computed and communicated distance abovesuch as by determining the difference in the position of the AGVs aswell as the length of any particular carrier and any system specificfactors for minimal allowable spacing. Therefore, each AGV may travel upto the designated spacing without interfering with the AGV ahead duringa stopped condition as each AGV controller 90 includes the distance tothe AGV ahead or prior AGV. In addition, as each AGV is able to detectany errors in spacing such that when the prior AGV restarts from astopped condition, a start delay may be calculated such that any restartis delayed until proper spacing exists. Similarly, if the spacing islarger than the desired minimal spacing, the subsequent AGVs mayincrease speed in a limited manner to approach the desired designatedspacing. Even in the event of a severe failure such as failure of thecommunication system or communication between each AGV, based upon thelast communicated position of a prior AGV, the subsequent AGV knows thesafe distance it may travel before requiring a stop. Therefore, if afailed vehicle is removed from a particular line segment, the data maybe immediately sorted such that the distances between the vehicles arereestablished without including the removed AGV and the signals to startmay be initiated from one vehicle and sent through other AGVs within theline segment 22.

The foregoing discussion discloses and describes an exemplary embodimentof the present invention. One skilled in the art will readily recognizefrom such discussion, and from the accompanying drawings and claims thatvarious changes, modifications and variations can be made thereinwithout departing from the true spirit and fair scope of the inventionas defined by the following claims.

What is claimed is:
 1. A method of operating a conveyor system formaterial handling in a facility, said conveyor system including aplurality of AGVs, each having a controller including a communicationsystem and a guidance system, said method comprising the steps of:determining a travel path having at least one line portion defined by aninitial line and an end line; inputting a guided route for the AGV tofollow, wherein the guided route includes instructions regarding theAGVs travel along the at least one line portion; assigning a unique IDto each AGV; guiding the plurality of AGVs along the travel path;automatically determining a lead AGV in the line portion; automaticallydetermining a last AGV in the line portion; automatically updating thelast AGV in the line portion each time an AGV from the plurality of AGVscrosses the initial line of the line portion; automatically updating thelead AGV to the immediate trailing AGV in the line portion each time thelead AGV from the plurality of AGVs crosses the end line; andmaintaining a designated spacing between each AGV from the plurality ofAGVs within the line portion.
 2. The method of claim 1 further includinga step of inputting the travel path, including the line portion, intoeach guidance system of the plurality of AGVs.
 3. The method of claim 1wherein said step of inputting a guided route for the AGV to follow alsoincludes inputting a desired line speed for the AGV to travel along theline portion.
 4. The method of claim 3 wherein said step of inputting aguided route for the AGV to follow also includes inputting a desiredtravel speed for the AGV when the AGV is not in the line portion andwherein the desired line speed and the desired travel speed are notidentical.
 5. The method of claim 1 wherein said lead AGV has a lead AGVspeed and wherein each of said plurality of AGVs following said lead AGVin the line portion match the lead AGV speed.
 6. The method of claim 1wherein said step of automatically determining a lead AGV includes thestep of assigning a lead token to the lead AGV.
 7. The method of claim 6wherein said step of automatically updating the lead AGV furtherincludes the step of passing the lead token from the lead AGV as itcrosses the end line to the immediate trailing AGV.
 8. The method ofclaim 1 wherein said step of automatically determining a last AGVincludes the step of assigning an end token to the last AGV.
 9. Themethod of claim 8 wherein said step of automatically updating the lastAGV in the line portion each time an AGV from the plurality of AGVscrosses the initial line of the line portion further includes the stepof passing the end token from the preceding last AGV to the updated lastAGV.
 10. The method of claim 1 wherein said step of automaticallyupdating the last AGV in the line portion includes the step of providingthe unique ID of the AGV updated as the last AGV to at least one ofanother AGV and a central controller.
 11. The method of claim 1 whereinsaid step of guiding the plurality of AGVs along the continuous travelpath includes the step of guiding the AGVs through the line portion. 12.The method of claim 11 wherein said step of guiding the plurality ofAGVs through the line portion further includes the step of guiding theplurality of AGVs with consistent spacing and speed in the line portion.13. The method of claim 11 wherein said step of guiding the AGVs throughthe line portion includes the steps of tracking the distance traveled bythe AGV since crossing the initial line to enter the line portion. 14.The method of claim 13 wherein after the AGV crosses the end line of theline portion the distance traveled is cleared from memory.
 15. Themethod of claim 1 wherein said step of maintaining a designated spacingbetween each AGV from the plurality of AGVs further includes the step ofdetermining the distance traveled from the initial line on the lineportion for each AGV.
 16. The method of claim 15 wherein said step ofmaintaining a designated spacing between each AGV further includes thestep of ensuring that the distance between each AGV is determined bysubtracting the distance traveled of a trailing AGV from a prior AGV andcomparing the determined distance between matches the designatedspacing.
 17. The method of claim 16 wherein said step of ensuring thatthe distance between matches the designated spacing further includes thestep of adjusting the speed of the trailing AGV to approach thedesignated spacing and repeating said step of subtracting the distancetraveled of the trailing AGV on which speed was adjusted from the priorAGV and adjusting the speed, and repeating the steps of subtracting andadjusting until the trailing AGV and prior AGV are spaced apart with thedesignated spacing.
 18. The method of claim 17 wherein said step ofadjusting the speed further includes the step of limiting any speedadjustment within a specified range.
 19. The method of claim 17 whereinas said step of adjusting the speed for AGV is performed, eachsubsequent AGV automatically in response to a speed adjustment by aprior AGV performs the steps of ensuring that the distance between eachAGV is the designated spacing and adjusting speed as needed to maintainthe designated spacing with the prior AGV.
 20. The method of claim 1wherein said step of maintaining the designated spacing further includesthe step of continuously calculating the distance traveled by each AGVfrom the initial line along the line portion and communicating saiddistance traveled to one of a prior and a trailing AGV.
 21. The methodof claim 20 wherein said step of maintaining further includes the stepof at least one of the prior and the trailing AGV calculating thespacing between the two AGVs and wherein the trailing AGV uses thecalculated spacing to adjust the speed of the trailing AGV to approachthe desired designated spacing.
 22. The method of claim 1 furtherincluding the step of stopping at least one AGV in the line portion inresponse to a stop condition and wherein the stopped AGV communicates astop status to each subsequent AGV within the line portion and whereineach subsequent AGV stops upon receiving said stop status from any priorAGV.
 23. The method of claim 22 wherein each stopped AGV determines thedistance to the immediate prior AGV.
 24. The method of claim 23 whereinthe leading AGV of the stopped AGV automatically restarts upon removalof the stop condition and communicates a restart signal to at least thetrailing AGV upon starting.
 25. The method of claim 24 wherein thetrailing AGV determines a start delay.
 26. The method of claim 25wherein said step of determining a start delay includes the step ofdetermining a distance between the prior AGV and the subject AGV and ifthe distance is greater than the designated spacing a start delay ofzero is determined and if the determined distance is less than thedesignated distance a start delay of sufficient time to ensure at leastthe designated spacing is maintained to the prior AGV is determined. 27.The method of claim 25 wherein each subsequent AGV performs the steps ofdetermining a start delay upon receiving a restart signal from the priorAGV.
 28. The method of claim 1 wherein said step of maintaining adesignated distance further includes the step of automaticallydetermining the length of at least one of a carrier and a load on theAGV and adjusting the spacing between the AGV to match the designateddistance while accounting for variations in length of the load andcarrier.
 29. The method of claim 1 further including the step ofmaintaining a communication link between each prior and each trailingAGV and stopping a trailing AGV upon failure of the communication link.30. The method of claim 1 wherein the travel path is a continuous loopand further including the step of directing the AGV along the travelpath to the next initial line after the AGV crosses the end line of thecurrent line portion.